Nitrided article of manufacture



Patented Nov. 27, 1934 UNITED s ATEs PATENT OFFICE NITRIDED OF MANUFACTURE Joseph V. Emmons, Shaker Heights, Ohio, as-

signor to The Cleveland Twist Drill Company, Cleveland, Ohio, in. corporation of Ohio No Drawing.

4 Claims.

ammonia usually being employed as the means for supplying the nitrogen. (See U. S. Patent No. 1,065,379 to Machlet.) It has been recognized, however, that a steel consisting of a simple alloy of iron and carbon does not absorb nitrogen during the customary nitriding process to an extent sufficient to make it useful as a body material for the production of articles having a nitrided case.

Dr. Adolph Fry made a very important contribution to this art by his suggestion to add alu- 0 minum to the alloy being nitrided in order to materially increase the rate of absorption of the nitrogen, thereby reducing the time required to secure sufficient penetration and further, a nitrided case secured in this manner is harder than a case possible on a simple alloy of iron and carbon. It has also been suggested (see Phillips U. S. Patent No. 1,697,083) to add molybdenum as an alloying element to the article being nitrided in order to make possible the rapid absorption of nitrogen and the production of a. satisfactory hard case of adequate depth. The nitrided surface or case formed on alloy steel articles of the type suggested by the prior art is usually satisfactory from the standpoint of its hardness and resistance to abrasion but a principal difliculty paid to the characteristics of the case supporting core, and the relationship between the core and its case has never been developed, especially such relationship as comprises my invention presently to be explained.

The prior art has been primarily concerned with the production of hard nitrided cases upon core materials which were too weak and plastic to withstand the high chip pressures encountered with many cutting tools or the concentrated loads frequently occurring in other forms of wear- Application February 8, 1933, Serial No. 655,881

resisting articles. This is particularly true because the temperatures at which the nitriding process is customarily e'flected (900 F. to 1100 F.) are such as to greatly reduce the hardness and strength and increase the plasticity of the steels customarily employed as core materials if they have been previously hardened. A tool formed with a plastic core of insumcient hardness and strength, even though provided with a satisfactory nitrided case of suflicient hardness and capable of withstanding abrasion will, nevertheless, be

useless for heavy duty work where the stresses to,

which the tool will be subjected are suflicient to deform the core to such an extent that the nitrided case will crack or chip.

The steels previously employed as materials for supporting a nitrided case have generally been low in alloy content with such compositions that even if hardened, they have but little ability to maintain such hardness and strength during the prolonged exposure to the high temperature required by the nitriding process (12 to 96 hours at 900 F. to 1100 F.). This peculiarity may be stated another way, by saying that the hard martensitic microstructure necessary to sufiicient hardness and strength, as above outlined, which can be produced in such steels by a drastic quench from a high hardening temperature, is so rapidly decomposed by tempering during the nitrlding process that it is largely converted to its softer and more plastic decomposition product, sorbite. The prior art has attempted to solve this problem by so distributing the stresses that concentrations in excess of the yield point of the core material are avoided. For many applications, particularly cutting tools and the like, this solution is neither convenient nor economical. In fact, for some possible applications, the avoidance of high concentrated stresses is not possible at all.

It is a principal object of my invention to provide an article of manufacture such as a tool or thelike having a hard and brittle nitrided surface and a supporting body of great strength and hardness such as is provided by a core of hard and relatively stable martensitic steel. It is a further object of my invention to provide cutting tools and other articles of manufacture with a surface layer or case having a hardness and resistance to abrasion greater than that of hardened tool steels such as high speed steel of the conventional 1841 type.

It is a further object of my invention to provide a supporting core underneath the hard surface layer of a nitrided case of greater resistance the description proceeds.

To the accomplishment of the foregoing and related ends, said invention, then consists of the means hereinafter fully described and particularly pointed out in the claims.

The following description sets forth in detail some methods and products exemplifying my invention, such disclosed procedure and products constituting, however, but some. of the various applications of the principles of my invention.

In general the previously stated objects of my invention are attained by providing for the body of a nitride hardened article a ferrous alloy containing such proportions of those alloying elements which are more soluble in alpha iron than in gamma iron that the alloy will retain a high degree of hardness and strength after exposure to such temperatures as are used in nitriding.

More specifically said body forming alloys may be provided by steels containing such :a proportion of said alloying elements, which are more soluble in alpha iron than in gamma iron as to so stabilize their martensitic condition that it will not be decomposed to a substantially sorbitic structure by such a tempering action as is prescut during the nitriding process. Tungsten, molybdenum, chronium and vanadium are examples of the elements more soluble in alpha'iron than in gamma iron, but it should be understood that no particular one of them is essential to the attainment of the desired objects. As will be seen by reference to the specific examples of compositions which are to follow, successful compositions have been observed in which each of the above named individual elements have been practically omitted.

It is recognized that such auxiliary alloying elements as manganese, silicon, nickel, cobalt, tantalum and uranium which are sometimes employed in ferrous alloys, may be also included in this body forming alloy if desired -for the effect usually produced by their presence.

In the attainment of the aforesaid objects ithas been found that certain tool steels employing these alloying elements can be caused to assume a very stable martensitic structure. It has also been found, that such steels in which this stable martensite has been developed have also the property of acquiring a hard case when treated by the conventional nitriding process. The above mentioned stable martensite may be p.0d1108d by heating the steels to the highest hardening temperature possible without causing a damaging amount of grain growth and by quenching with suitable rapidity such as in an oil bath. Where such treatment produces an austenitic structure, it can be converted to a martensitic structure by tempering, if desired, during the nitriding process. This stable martensite offers greater resistance to tempering action than the type of martensite produced in previously known steels for nitrided articles.

The degree of this stability can be so varied by different proportioning of the above mentioned alloying elements that any desired amount of martensite or its first decomposition product, troosite, can be retained in the core after the nitriding operation. This control of the microstructure makes possible the control of the hardness, strength and plasticity of the body material which will be called upon to give adequate support to the nitrided case during the subsequent use of the tool or other article of manufacture.

A preferred example of a satisfactory composition of alloy steel employing the above elements and which has been found suitable for attaining the enumerated objects is as follows:-

Example I Per cent Carbon about 0.70 Tungsten about 18.00 Chromium about 4.00 Vanadium about 1.00

the balance being iron together with such other alloying elements and impurities as are usual in alloy steels.

The above analysis will be recognized as a type analysis of a standard high speed steel of commerce. When treated by the usual nitriding process this steel has been found to take a nitrided case of great hardness. A hard case is produced whether the high speed steel is processed in the annealed or in the hardened condi-- tion. The hardness of the case'on the hardened high speed steel has been observed to be very high, in fact higher than the cases produced on some previously known nitriding steels by the same treatment. The supporting body to which the nitrided case is applied, if it has been previously hardened, maintains a high degree of hardness after the nitriding process. In fact, it has been found possible to perform the operation of tempering the high speed steel which is usualy done at a temperature between 1000" and 1100 F. simultaneously with nitriding. The microstructure of the high speed steel core after hardening and nitriding consists of mar tensite with a small amount of troosite. which is the normal structure of hardened and tempered high speed steel as employed for cutting tools. This type of high speed steel is useful for the bodies of such tools as drills and reamers upon which surfaces subject to abrasion have been nitrided:

Another example of a specific alloy composition which has been characterized by excellent results contains the following:-

the balance being iron, together with such impurities as are usual in alloy steel. Such a steel withstands hardening temperatures as high as 2000 F. without a prohibitive coarseness of the grain and retains hardness and cutting quality after drawing temperatures as high as 1100 F. It is also much more readily machined than known alloy steels containing high silicon. Ar-

ticles-made up of such alloys are readily nitrided and because of the ability to withstand a drawing temperature above that at which nitriding is effected, it becomes possible to apply a nitrided case to a hardened part and still retain in it a usable degree of hardness, strength and cutting quality. This unique result makes particularly possible new combinations of properties in such articles as tools and bodies and auxiliary wearing and cutting parts of composite tools, for instance, those utilizing a hard metal alloy such as tungsten carbide.

. Still another illustrative specific composition is as follows;-

Example III Per cent Carbon about 0.61 Manganese i about 0.23 Silicon -1 about 0.29 Chromium about 3.65 Tungsten about. 1.69 Vanadium about 1.00 Molybdenum about 6.67

the balance being iron, together with such impurities as are usual in alloy steels. I This steel possesses properties which approac those of high speed steel, although the amount of alloy present is much less than is usually employed in such steels. With this composition a hardening temperature as high as 2200" F. may be employed together with drawing temperatures up to 1150 F. Tools made from this steel such as twist drills, possess great strength and cutting quality, these properties being maintained at op crating temperatures up to red heat.

After hardening at 2200 F. and nitriding a steel according to the last above enumerated composition has a surface layer of great hardness supported by a rigid core of great strength and of a hardness sufllcient to itself function as a cutting tool.

Still another example of a specific alloy steel composition which is, suitable for body material for nitrided articles is as to1lows:--

Example IV Per cent Carbon about 0.66 Chromium about 4.26 Tungsten about 0.10 Vanadium about 1.14 Molybdenum about 10.19

the balance being iron, together with such other alloying elements and impurities as are usual in alloy steels. This steel illustrates a type in which molybdenum is the principal alloying element and which is practically free from tungsten. It also develops a useful nitrided case while maintaining a highdegree of hardness and strength in the underlying steel of the body.

A still further example of a specific alloy steel composition successfully utilized for the purposes- 'of this invention is as follows:-'

Example V Per cent Carbon about 0.77 Chromium about 3.85 Tungsten about 1.99 Vanadium about 1.03 Molybdenum about 11.15 Cobalt about 9.06

the balance being iron and such other alloying elements and impurities as are usual in alloy nealing conditions.

steels. The steel of the last above mentioned composition is of the type in which molybdenum is the principal alloying constituent and contains about one fourth as much tungsten as molybdenum. It also contains a considerable percentage of cobalt. The body of this steel underneath the nitrided case has exceptional hardness and strength due to its great resistance to tempering. Its microstructuraafter nitriding, is still predominantly martensitic'.

Another example of a specific alloy steel composition is as follows:-

Example VI 1 Per cent Carbon. about 2.10 Manganese ;.about .29 Silicon about .66 Chromium about 12.10 Vanadium about .22

the balance being iron together with such other alloying elements and impurities as are usual in alloy steels.

This steel takes a thin but very tough case which is particularly useful for certainwear resisting tools, such as arbors or mandrels. Its microstructure after nitriding is predominantly tr'oosite.

Another example of a specific alloy steel composition is as follows:

Example VII Per cent 1 Carbon about .66 Manganese about .28 Silicon -1 about 1.20 Chromium -1 about .10 Tungstenuu about 12.08

I .Vanadium about .65 Molybdenum--. about 1.63

the balance being iron together with suchother alloying elements and impurities as are usual in alloy steels.

This steel is notable on account of its very low chromium content. It has been observed to take a deep, hard nitrided case of excellent gradation into the core when it had been previously hard ened. When nitrided in the annealed condition 1 it did not take a hard case.

Another example of a specific alloy steel composition is as follows:-

Emample VIII Per cent Carbon. about .87 Manganes ut .12 Silicon about .40 Chromium about 3.35 V i m about 1.31 Molybde about 7.65 Nickel"- about 2.17

Example IX I Per cent Carbon .57 Manganese .26 Silicon .27

Chromium 3.96 Tungsten 18.69 Vanadium None Molybdenum None the balance being iron together with such other alloying elements and impurities as are usual in alloy steels. I

This steel is notable for the absence of both vanadium and molybdenum. It takes a nitrided case of great depth and hardness.

Example X Per cent Carbon 1.29 Manganese .11 Silicon .47

Chromium 3.29 Tungsten 1.61 Vanadium 3.29 Molybdenum 8.04

the balance being iron together with such other alloying elements and impurities as are usual in alloy steels.

This steel is notable forhigh contents of carbon and vanadium. It takes a nitrided case of great depth and hardness.

As previously indicated, the compositions enumerated herein are such that the alloying elements are present in such proportions as to stabilize the martensitic structure of the steels so that it is possible to temper the steel simultaneously with the nitriding operation without losing a prohibtive amount of hardness and strength.

The compositions hereinbefore enumerated are sufliciently hard and of such great strength so that when tools and like articles made therefrom are subjected to severe working stresses the supporting core for the nitrided case will not be deformed and the case will accordingly be preserved against cracking and chipping.

The utility of such nitrided cases is not confined to the cutting portion of tools but may be widely extended to such purposes as pilots, guides, or other articles and parts which are subject to abrasion or wear.

As an example, nitrided. parts so supported may be used in complex' tools of which the principal cutting or wear-resisting action is performed by parts of non-ferrous, hard metal compositions such as tungsten or tantalum carbides. When so used, it has been found that the nitriding operation can be carried out without material damage to the working parts of the hard metal composition and with actual improvement of the strength and toughness of the tool body due to the desirable tempering action.

Other modes of applying the principle of my invention may be employed instead of the one explained, change being made as regards the product and method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

I I, therefore, particularly point out and distinctly claim as my invention:

1. A steel article of manufacture having a hard nitrided surface layer supported by an underlying core of predominantly martensitic steel and which martensitic structure is rendered stable to withstand the tempering eilect of the nitriding operation by the inclusion in the steel of about 0.10% to about 12.10% chromium and at least one of the following elements in the proportions named, viz., tungsten about 0.10% to about 18.69%, molybdenum about 1.55% to about 11.15% and vanadium about 0.22% to about 3.29%.

2. A steel article of manufacture having a hard nitrided surface layer supported by an underlying 105 core of predominantly martensitic steel and which martensitic structure is rendered stable to withstand the tempering eifect of the nitriding operation by the inclusion in the steel of about 0.10% to about 12.10% chromium and about 110 0.10% to about 18.69% tungsten.

3. A steel article of manufacture having a hard nitrided surface layer supported by an underlying core of predominantly martensitic steel and which martensitic structure is rendered stable to 115 withstand the tempering effect of the nitriding operation by the inclusion in the steel of about 0.10% to about 12.10% chromium and about 1.55% to about 11.15% molybdenum.

4. A steel article of manufacture having a hard 12o nitrided surface layer supported by an underlying core of predominantly martensitic steel and which martensitic structure is rendered stable to withstand the tempering effect of the nitriding operation by the inclusion in the steel of about 125 0.10% to about 12.10%. chromium and about 0.22% to about 3.29% vanadium.

JOSEPH V. EMMIONS. 

